This invention is related to a probe contactor having a plurality of buckling beams, and more particularly to a probe provided with self-scrubbing features such that when a force is applied to the probe, the tip of the probe changes its landing angle, providing a controlled scrub by wiping action.
Present trends in the microelectronic industry portend ever increasing chip densities, which translate in the need for new probing devices to accommodate the increase in the number of I/O pads.
When testing the electrical characteristics of an integrated circuit, whether a chip, module, and the like, the probes of a contactor engage the pads of the package under test to provide an electric path from the device under test to the tester. Accordingly, it is imperative to ensure that the probes contacting the pads be endowed with a controlled force to prevent or at the least, reduce, any damage to the pads. Oftentimes, excessive force exerted by the probe on the pad, particularly when dealing with a chip, can destroy the pad altogether, requiring the additional step of having to reconstruct the pad in question by way of techniques known as solder reflow.
Present art probes include etched lead frame cantilever contacts, which are highly versatile and which are mainly used for peripheral footprints. With the advent of more complex footprints, commercial cantilever contactors became more prevalent. These contactors are characterized in that a popout card is plugged into a motherboard. The contacts of such devices typically take the form of discrete wires extending outwardly and secured around the periphery. The distinct advantage of such probes is its ability of serving the dual purpose of acting as a space transformer with the motherboard acting as a mounting platform for the desired discrete components.
Cantilever probes are oftentimes provided with several layers of contacts in order to adapt such contactors to pads in an array formation. Such probes with more than one layer are difficult to design, specifically having a plurality of probe tips to be coplanar for allowing contact to all the pads of an array with equal force. Even when multiple layers of probes are used, it is still difficult to physically position them without they interfering with each other.
The aforementioned commercial cantilever contactors were followed by custom probes, particularly useful for array footprint, become prevalent in view of their ability of incorporating a vertical acting motion. Both types, the cantilever contactors and custom probes provided electrical contact coupled to a spring action that proved highly beneficial to account for product height variations. More details can be found in an article published by the International Journal of Microelectronics and Electronic Packaging, Vol. 15, No. 4, Fourth Quarter 1992 (ISSN 1063-1674), pp. 229-238, entitled xe2x80x9cProbing considerations in C-4 Testing of IC wafersxe2x80x9d.
A prior art buckling beam contactor is shown in FIG. 1. It consists mainly of a housing, insulating dies with an area array of holes, and an array of contacts or wires. These contacts employ the principal of a buckling column whereby the application of a force beyond the critical force causes buckling to occur. Contact lengths are 16.6 mm for 0.10 mm diameter contacts. Contacts are made of paladium allow and are coated with insulating parylene. They further have swages head which interfaces with gold pads on the footprint transformer. The upper and lower dies provide both location and bearing surfaces. The offset die provides both bias prebuckle and vertical retention. The contacts traverse all three dies using holes that are aligned with respect to each other and which are positioned to coincide with the footprint of the device under test.
In present day probes contactors, deflection of a probe is proportional to the amount of force exerted on the probe. This is particularly true since conventional probes are designed for only vertical motion, to a point that these probes are oftentimes referred to as vertical probing technology.
Another type of buckling beam extensively used in the industry and known as a Cobra probe, is characterized as a hybrid type of buckling beam. The housing, insulating dies with an area array of holes, and the array of wires, are the same as those previously described with reference to FIG. 1. The Cobra probe differs in the design of the contact or wire. The wire is best described as a hinged-offset hybrid column, typically 6.4 mm in length and using a 0.13 mm Paliney 7 wire. The main difference resides in the wire being flattened and precurved to form what is known as the active portion of the contact. The head is swagged at 90 degrees with respect to the active portion. Conventional Cobra probes do not have an offset die, with the lower die being displaced with respect to the fixed upper die by 1.3 to 1.9 mm. The contact shank rides essentially vertically in the lower die. A limited amount of wiping (scrubbing) of the pads of the device under test is achievable, although the construction of the Cobra probe precludes an effective wiping action which is essential to establish good contact with the pads because of its characteristic low inertia and vertical retention.
The problem associated with vertical only movement probe is that when probing is performed on Al pads, as typically found in semiconductor wafers, the probe does not easily penetrate and push off the oxide layer on the pad, resulting in a high and unstable contact resistance.
Related patents are:
U.S. Pat. No. 4,027,935, Contact for an electrical contactor assembly, issued to H. P. Byrnes et al., and of common assignee; and
U.S. Pat. No. 4,423,376, to H. P. Byrnes et al., Contact probe assembly having rotatable contacting probe elements, and of common assignee.
Accordingly, it is an object of the present invention to provide a buckling beam probe, wherein after a critical buckling point, the probe force remains constant.
It is another object of the invention to provide a buckling beam probe having the benefits of a cantilever probe, i.e., by providing scrub action while probing.
It is yet another object of the invention to provide a buckling beam wherein the scrubbing direction and scrub length can be accurately controlled by design, independent of the amount of compression, thereby allowing the level of the device under test relative to the probe head, and the amount of probe compression to be controlled precisely by the test system.
It is still another object of the invention to provide a shifty self-scrub design, wherein the scrub direction changes during testing, such that under no load, the tip points to the right which is set by the shift of the bottom plate, and when the probe is under load, the tip scrubs to the left, which is set by the offset guide plate.
It is a further object of the invention to provide a probe beam using straight wire materials which are not highly ductile, e.g., tungsten, as opposed to a Cobra probe which body is normally flat, pre-bowed shape and wherein the probe head is normally coined or formed to a ball shape.
It is still a further object of the invention to provide a probe beam which can be easily repaired and wherein individual probe beams can be replaced without removing guide plates, thereby dispenses the need for a probe tip alignment after replacing the beam.
In a first aspect of the invention, there is provided a buckling beam contactor for contacting an array of pads positioned on a device under test that includes: a top, an offset and a lower insulating die, each of the insulating dies being provided with holes, each of the dies being separated from the next die by an insulated spacer of variable thickness; and an array of flexible wires positioned substantially perpendicular to the dies, each of the flexible wires crossing a corresponding hole in each of the top, offset and lower dies to allow each wire respectively contact a pad of the device under test, wherein by shifting the center of the hole of the lower die relative to the center of the offset die, the tip of the wire exits from the lower die at an angle with respect to the plane formed by the pads of the device under test.
In a second aspect of the invention, there is provided a method of providing of contacting an array of pads positioned on a device under test by way of a buckling beam contactor, the method including the steps of: supplying a top, an offset and a lower insulating die, each of the insulating dies being supplied with holes, each of the dies being separated from the next die by an insulated spacer of variable thickness; and providing an array of flexible wires positioned substantially perpendicular to the dies, each of the flexible wires crossing a corresponding hole in each of the top, offset and lower dies to allow each wire respectively contact a pad of the device under test, wherein by shifting the center of the hole of the lower die relative to the center of the offset die, the tip of the wire exits from the lower die at an angle with respect to the plane formed by the pads of the device under test.
The exit angle of the wire tip is controlled by the relative displacement of the offset die relative to the bottom die, such that the exit angle of the tip of the wire at the bottom die changes when the probe wire is under compression.
When a reciprocating motion to the tip of the wire is applied when the wires contact the surface of the device under test, a scrubbing motion is achieved that lowers the resistance between the pad of the device under test. In this manner, the tip of the wire cleans the surface of the pads and prevents contaminants from adhering to the tip of the wire.